1. Field of the Invention
The present invention relates to a shadow mask for a color CRT (Cathode-Ray Tube), in particular to the shadow mask for the color CRT which is capable of preventing a distortion phenomenon (uneveness) of a screen due to outer vibrations and impacts by improving stiffness of the shadow mask.
2. Description of the Prior Art
In general, a shadow mask installed at the rear end of a panel of a color CRT (Cathode Ray Tube) performs a function for getting each electron beam radiated from a electron gun thrown accurately each color of a fluorescent screen corresponding to the electron beam, particularly it can be shaken due to outer vibrations and impacts and can be directly damaged by the outer vibrations and impacts. Accordingly, strengthen stiffness of the shadow mask is required in order to get high visibility of the screen.
However, it is impossible to form beads on an effective surface of the shadow mask different to other parts, but it is possible to form the beads on an ineffective surface as a framework of the shadow mask in order to improve the stiffness of the shadow mask.
As depicted in FIG. 1. and FIG. 3B., the conventional shadow mask will now be described.
The conventional shadow mask comprises a front panel 1 installed on the front side of the color CRT, a three color fluorescent screen 1a having a red R, green G, blue B color installed on the inner side surface of the panel 1, a shadow mask 5 which is supported by a frame 6 and is installed on the rear portion of the panel having a certain distance from the fluorescent screen 1a in order to make an electron beam 10 throw electron beams accurately on the fluorescent screen 1a, the frame 6 supporting the inner upper and lower end of the panel 1 by a spring 4, an inner shield 11 combined to the rear end of the frame 6 in order to prevent electron beam 10 from being influenced by an outer magnetic field.
The red, green, blue colors electron beam 10 radiated by an electron gun 3 is deflected toward the fluorescent screen 1a by the vertical and horizontal deflection magnetic field of a deflection yoke 8, is subdivided by passing through electron beam through holes of the shadow mask, and illustrates an image by being thrown on the fluorescent screen 1a. 
Meanwhile, the electron beam 10 radiated by the electron gun 3 has to be accurately thrown the fluorescent screen 1a corresponding to the each color, when the outer impact and vibration are transferred to the shadow mask 5, the electron beam 10 can not be thrown accurately on the fluorescent screen 1a in accordance with the each color, but is deviated, and wave figures occur on the fluorescent screen 1a. 
This is called as a microphonic phenomenon, in order to prevent directly the phenomenon, the stiffness of the shadow mask 5 has to be improved.
In addition, a drop impact experiment is performed on the color CRT, when there is distortion of the shadow mask as above, the screen 1a is uneven, accordingly credibility of the product lowers.
In order to decrease distortion of the shadow mask 5, as depicted in FIG. 3, a hemispherical bead 15 having hemispherical cross section (A-Axe2x80x2) can be formed on the ineffective surface as the circumference of the effective surface 16 of the shadow mask 5, or as depicted in FIG. 3B, a stepped bead 15 having stepped cross section (B-Bxe2x80x2) is formed.
A non described reference numeral 5a illustrates a skirt portion combining to the frame 6.
According to the conventional technology, the hemispherical bead or the stepped bead 15 is formed on the ineffective surface of the shadow mask 5, but it does not actually improve the stiffness of the shadow mask 5.
Meanwhile, FIG. 4. illustrates shapes of major vibration modes. Herein, when the ineffective surface of the shadow mask 5 moves up and down due to the outer vibration, the skirt portion 5a moves naturally left and right as depicted in FIG. 5.
In the portion corresponding to a certain length 1, the relative vibration ranges of the effective surface 16 and skirt portion 5a are in proportion to a section modulus I corresponding to the length 1. In other words, when the section modulus is getting bigger, the relative vibration ranges of the effective surface 16 and skirt portion 5a are getting smaller, and the stiffness of the shadow mask 5 is getting improved.
On the contrary, as depicted in FIG. 5., when there is no bead on the ineffective surface, the section modulus I of the ineffective surface can be illustrated as below.                     I        =                              It            3                    12                                    [Equation  1]            
Thickness of the most shadow masks 5 is thin, accordingly the section modulus value is small, accordingly the relative vibration occurs easily.
Herein, in order to improve the stiffness of the shadow mask 5, the relative vibration has to be prevented by keeping a certain angle (xcex8(t)) between the effective surface 16 and skirt portion 5a. 
The bead shapes depicted in FIG. 3A and FIG. 3B influence the section modulus I so as to be small, accordingly the beads according to the FIG. 3A and FIG. 3B can not help increase of the stiffness of the shadow mask 5.
It is an object of the present invention to provide a shadow mask for a color CRT which is capable of increasing an impact character and a vibration character by strengthening stiffness of the shadow mask by improving shapes of beads formed on an ineffective surface of the shadow mask so as to increase a section modulus.
In order to achieve above mentioned objects, the shadow mask of the present invention comprises an effective surface having electron beam passage holes all over, an ineffective surface having a certain width surrounding circumference of the effective surface, a skirt portion curved formed on the circumference of the ineffective surface, and concave-compressed beads formed on the ineffective surface along the skirt portion direction in order to improve the stiffness of the effective surface.
In the concave-compressed bead of the preferred embodiment of the present invention, it is advisable to form a plurality of the concave-compressed beads along the circumference of the shadow mask with a certain interval.
In addition, it is advisable to form the plurality of the concave-compressed beads sequentially along the circumference of the shadow mask.
In the other embodiment of the present invention, it is advisable to form a stepped bead having at least two steps.
In addition, when overall length of the ineffective surface is L1 and length from a dotted line to a concave-compressed bead forming portion is L2 (ref. FIG. 8), it is advisable for satisfying the range 0.3 less than L2/L1 less than 0.8.
And, when overall length of the skirt portion is H1 and length of the concave-compressed bead on the skirt portion is H2 (ref. FIG. 8), it is advisable for satisfying the range 0.3 less than H2/H1.